LIGO/Virgo S191216ap: Two candidate counterparts from UKIRT/WFCAM z-band observations

As part of our search for strongly-lensed gravitational waves and their electromagnetic counterparts, we observed a contiguous area of 0.75 square degrees within the sky localization of the gravitational wave trigger S191216ap (GCN #26454) with the WFCAM instrument on UKIRT through the z-band. The observations were centered on the position of the HAWC sub-threshold detection of gamma-ray flux reported by Martinez-Castellanos (GCN #26472), and encompassed the full extent of their 68% confidence region. The observations comprise two epochs: Epoch Date(UT) Start(UT) End(UT) Airmass FWHM(arcsec) 1 2019-12-20 04:45:21 05:45:57 1.3-1.7 1.2-1.6 2 2019-12-21 04:31:26 05:43:34 1.3-1.8 1.6-1.8 We estimate that Epoch 1 reaches a 5 sigma point source sensitivity of AB~21.5. Epoch 2 is less sensitive than Epoch 1, with noticeable variations in Epoch 2 between the four observations required to achieve contiguous coverage with WFCAM. A preliminary comparison of Epoch 1 with templates derived from Pan-STARRS1 data (nominal sensitivity of AB=22.3) identified two candidate transient sources. Neither source is known to the Transient Name Server, and Minor Planet Center. The celestial coordinates and estimated z-band apparent AB magnitudes of these two candidates are: Name RA(J2000) Dec(J2000) AB(Epoch1) AB(Epoch2) GLGWc19a 21:32:45.50 +05:19:58.0 20.8+/-0.1 ~22 GLGWc19b 21:35:20.11 +04:55:19.8 20.6+/-0.1 >20(tentative) Neither source is associated with an obvious candidate gravitational lens (i.e. massive galaxy, group or cluster) to the depth of the PS1 data. However, GLGWc19a is 7 arcsec West, 1 arcsec North of the center of an edge-on disk galaxy (possible host?) that is located at 21:32:45.97 +5:19:57.0. These sources are observable at Airmass<2 for ~30 minutes following the end of evening twilight in the Northern Hemisphere in the next few nights, before becoming unobservable through the winter months. We therefore encourage urgent follow-up observations of GLGWc19a, GLGWc19b and the candidate host galaxy adjacent to GLGWc19a. We thank the Director of UKIRT, Klaus Hodapp, for quickly approving our urgent DDT proposal. We are also grateful to Mike Irwin for kindly expediting the processing of our data at Cambridge Astronomy Survey Unit (CASU). This circular is citable.